Formyl-substituted porphyrinic macrocycles provide versatile intermediates and target molecules in bioorganic and materials chemistry. Notable reactions of the porphyrinic formyl group include classical reactions of aldehydes (e.g., Wittig,1-3 Grignard,2,4 McMurry,5 Schiff's base,6-8 Knoevenagel7,9)10 as well as reaction with pyrrole or a dipyrromethane leading to multi-porphyrinic architectures.11 The formyl group also has been exploited in supramolecular chemistry wherein the oxygen of the formyl group binds to the apical site on a neighboring metalloporphyrin.12 Although a few formyl-porphyrinic compounds occur naturally (e.g., chlorophyll b), most must be synthesized de novo. The generic method for introducing a formyl group to a porphyrinic macrocycle entails Vilsmeier formylation.10 Vilsmeier formylation, either with the traditional DMF/POCl36,13 or the more recent HC(OMe)3/TFA or SnCl4,3 can only be carried out with metalloporphyrins that are stable toward strong acids (e.g., copper or nickel chelates). Hence formylation typically requires three steps: (1) insertion of copper into a free base porphyrin, (2) formylation of the copper chelate, and (3) demetalation of copper to give the free base porphyrin bearing the formyl group. The removal of copper generally requires strongly acidic conditions such as TFA in H2SO4. The yield of the Vilsmeier formylation is typically quite high (though mixtures of polyformylated metalloporphyrins are known10,14,15). However, the requirement for a three-step procedure, use of strong acid, and limited control over the site of formylation presents obvious limitations.
There exists a need for a milder and more direct procedure for preparing formyl porphyrins. Two routes to porphyrins bearing distinct patterns of meso substituents include (1) the self-condensation of a dipyrromethane-1-carbinol affording trans-A2B2-porphyrins,16 and (2) the reaction of a dipyrromethane and a dipyrromethane-1,9-dicarbinol affording porphyrins with up to four different meso substituents (ABCD-porphyrins).17 